1. Field of the Invention
This invention generally relates to a navigation system which uses satellites to compute position and velocity data for guiding or tracking a variety of airborne, marine, and land-based systems.
2. Description of Related Art
The accuracy of position-determining systems for airborne, marine, and land-based systems has been greatly increased with the advent of satellite technology, and in particular the Navigation Satellite Timing and Ranging Global Positioning System (NAVSTAR GPS). In this system, a total of 24 satellites are deployed, with four satellites in each of six orbit planes, so that at least four satellites come within the same field of view twenty-four hours a day all over the world.
The general approach to utilizing GPS for navigation purposes, known as absolute GPS, is simply to calculate the range to the satellites in view by measuring the transmission and receiving time in comparison with a clock synchronized to the satellite clock given the position of the satellite at the transmitting time. The accuracy of this approach is approximately 15 meters spherical error probable (SEP). In many applications, such as position and velocity determination for the purpose of assisting precision approaches during landing of aircraft, an accuracy of 15 meters is insufficient.
In an attempt to improve the accuracy of GPS in general, a technique known as differential GPS has been proposed to eliminate those errors which are common to both a user and the reference receivers. Differential GPS uses a GPS receiver at a fixed surveyed site to compute fixed bias corrections for the GPS satellite range measurements based on the difference between the position calculated using the reference receiver's GPS measurements and that defined by the site survey. These bias correction measurements are then communicated via a data link to a vehicle whose position and/or velocity is to be determined.
While for many applications differential GPS navigation represents an improvement over absolute GPS, it too has a number of drawbacks. The need for a fixed survey site and a communication link between the site and the subject vehicle, as well as the latency and degradation of the corrections with time, all limit the ability of differential GPS to provide the very accurate position data required for precision applications such as approach and landing navigation requirements at airports.
A variation of the differential GPS technique, in which the GPS signal received at the subject vehicle is retransmitted to the base site for comparison, has been proposed in U.S. Pat. No. 5,119,102. While similar to the present invention in its use of retransmission, the designers of this system have apparently failed to appreciate the possibility of real-time comparison of simultaneously received direct and retransmitted GPS signals.
The core of their approach, receipt of the GPS signals, time tagging with "Time of Arrival," recording of the GPS data, and subsequent transmission of the data at a slower rate via a radio channel, are all unnecessary and wasteful. In contrast, this invention dispenses with all these elements and provides a completely different approach to re-transmission which will provide for substantially greater efficiency and error reduction.
The need for accurate navigation data is especially acute in the case of automatic carrier landings, where the runway is exceedingly short and narrow and extremely unforgiving. The slightest miscalculation in the navigation of the aircraft relative to the carrier runway could result in disaster. Present systems, including absolute and differential GPS, and the system disclosed in U.S. Pat. No. 5,119,102, are incapable of meeting the accuracy requirements for such carrier landings.